Acoustic device



Oct. 8, 1929. H c, so 1,730,425

ACOUSTIC DEVICE Filed Octv 11. 1927 4 Sheets-Sheet l m ma ATTORNEY Oct. 8, 1929. H. c. HARRISON ACOUSTIC DEVICE Filed Oct. 11, 1927 4 sheets sheet 2 Filed Oct. 11, 1927 4 Sheets-Sheet Oct. 8, 1929. H. c. HARRISON ACOUSTIC DEVICE Filed Oct. 11. 1927 4 SheetsSheet 4 :lw 38 "z Wmvmq /7NR [2 /14 /s0/v 4%, W ATTORNEY Patented Oct. 8, 1929 unrrso srras r HENRY C. HARRISON, OF PORT WASEINGTON, NEW

'r'r QFFECE YORK, ASSIGNOR 'ro WESTERN ELECTRTCCOMPANY, INCORPORATED, OF NEW YORK, N. Y., ACORPORATION OF NEW YORK ACOUSTIC DEVICE Application filed October 11,1927, Serial No..225,500, and-in Germany- October21, 1923.

. ,This invention relatesto sound wave transmission systems, and particularly to sound translating devices, such as phonographs and electrical loud speakers, in which sound wave energy is'transmitted as mechanical vibrations, The presentapplication is continuing as to the subject matter-of my co-pending application Serial No. 610,977, filed January 6, 1923, and as to certain parts of my co-pending applications SerialNos, 628,168, filed March 28, 1923, and 33,619, filed May 29, 1925. The broad principles of the invention and their application, as hereinafter described, .to sound converting systems in general are disclosed in the said coending application SerialjNo. 610,977 theparticular embodiment of-the invention in a stylus-operated phono graph, also-described herein, is disclosed'in my co-pending application Serial N 0. 33,619; and theacoustic features of the sound amplifying h0'rn',-which forms part of the present invention, are disclosed. in my co-pending ap lication Serial-No, 628,168.

n object of the invention is to improve the response-frequency characteristic of sound translating systems, wvliereby sound wave energy of all frequencies in the important range of speech and musical tones may be efliciently transmitted, and the sound waves faithfully recorded or reproduced. Another object is to improve the coupling between the mechanical portion of a sound translating system and the air, so that conversion of the wa-ve'energy is effected with uniform high efficiency at all frequencies. A corelated object is to effect in an improved manner the damping of resonance vibrations in the mechanical system of an acoustic translating device by the energy radiation properties of the sound radiating means.

In acoustic systems the translation of sound wave energy into other modes of vibration is most commonly effected'by means of a vibrating diaphragm, 0r analogous device, which is set in motion by the impinging sound waves, or which is mechanically driven and imparts its energy to the air as sound waves. :The diaphragm may be arranged to radiate or absorb sound wave energy direct- 1y, or it may be-ooupled to the free air by meansof a tapered horn. The mechanical system attached to-the diaphragm may in turnbe coupled to. an. electromagnetic driving .or converting means, or it may be connected to a mechanical source of vibrations, for example, a stylus engaging the groove of a phonograph record. The system asa whole, whatever the particular arrangement may be, constitutes a wave transmission line along which the vibratory Wave energy is transmitted in accordance wit-lithe general laws of wave propagation. Due;to.the massesand elasticities of the component parts, the mechanical portionof the systemv as arule possesses one or more natural periods of vibration, in consequenceof which the frequencyresponse characteristic for forced vibrations is markedly distorted by resonance at these frequencies. g v In accordance with thisinvention the vibrating system of an. acoustic translatingdevice is treated as a wave transmission line of composite-character, partly mechanical. and partly acoustical. The distortion of the frequency-response characteristic due to resonance in' recognized to be videntical with distortion due to wave reflection, and is eliminated by so arranging and proportioning the elements of the system that waves of all frequencies in the desired range are transmitted through the system from end to end without suffering reflection. Each. portion of the line is adapted to have the-inherent capability of transmittin all of the desired frequencies without su stantial .loss. In addition, in order thatthe inherent'transmission characteristics may be fully developed and utilized, the characteristic impedances of the several portions are matched to each other and to the impedance of the radiating device, which constitutes theterminal load for the system. The matching of the impedances may be achieved by making the different portions of the line similar with respect to their mechanical properties, or it may be effected by the introduction of suitable impedance transformation means.

An essential element of the system is a sound radiator capable of radiating wave energy with uniform high efiiciency at all frequencies in the speech or musical range. Such a radiator provides a terminal-load for the mechanical line which has a substantially constant and resistive impedance, and if the im pedances of the other portions of the line are properly matched thereto, the constant resistance becomes characteristic of the system as a whole.

- .Itlhas already been noted that distortion due t-ofwa-ve'rieflecti'on may be identified'xvith distortion due to resonance in the vibrating system. combination, the matching of the various linesections to each other has the-efi'ct of distributing the natural periods of .vibration more or'less uniformly throu h the transmis sion range, andof making them susceptible of uniform-'dampingby means of a single resistive element at the end' of the line; Matching the resistance .of the radiator to the impedances of the line sections provides just the correct amount of" damping to make the response uniform-at all frequencies in the trans mission range. i

The term impedan'ce'is here used in its general sense to define the ratio of an applied simple'harmonic vibratory force to the resulti'ng' elocity of the system or body to which the force isapplied-i In this sense itis applicable to vibration systems of all types regardless- 'of-' the nature""'of'ftheimedium or the character of the force -The characteristic impedance of a line is defined as the impedance of the line when ,it is assumed to' be infinitely extendedi *This quantity" is descriptive of the properties 'of any shortsectien of the line, although 'it does not by itself determine allof the properties. If the line is not uniform'but is made up of-c'oupled sections' of lumped impedance elements, for instance a chain of masses coupledby springs, each section of the-line may have 'itsown characteristic impedance, which is defined as the impedance of a line made upof an infinite number of recurrent sections of that particular type. Lines of the lumped impedance t ne, as distinguished from uniform lines, are markedly selective in their wave transmission characteristics, but by arranging the coupling of the elements in a particular manner and proportioning them in accordance with prescribed impedance values,; suchlines'may be adapted totransmit freely \va esof all frequencies within a predetermnied wide range Structures of this type are known as broad band wave filters. For a more complete discussion of their propertiesreference is made to the Bell System Technical J ournal, Vol. I, No. 2, Physical theory of the electric wave filter, by G. A. Campbell.

In the preferred embodiment'of the inventi on. hereinafter described in detail, the sound radiating means consists'of a tapered horn in iclrthe'cross sectional area of the conduit varies according to an exponential law with Regarding the system as a resonant distancealong the axis. The horn is ener gizcd by, or imparts: energy to, a diaphragm coupled to the small end by an air chamber in the usual manner. A feature of the invention relates to the proportions of the air chamber and the diaphragm whereby the elasticity of the included ir is co-ord-inated with the impedance coetiicieuts of the other line elements, and whereby the impedance ofthe horn is matched to the impedance of the mechanical system. Y

Other features relate to the construction of the hcrn, particularly with regard to the form and proportions of the sound conduit, whereliy the range of uniform transmission and radiation efficiency is extended downwards to include the'low'est audiblefrequen cics. I Additional features are concerned with particular constructions for the elimination of undesired elastic and frictional-restraints in the mechanical system, whereby the re spouse is maintained ata uniform level down to the lowest frequencies'of speech, andwith methods for increasing the rigidity and lightness of the elements, .so 'that the response characteristic is not impaired at high frequencies. i

In itsoroader aspect the invention is'not restricted to any particular typeof radiator, but contemplates, rather-,- constructions whereby the flow of energy throrwlrthe sys tem and into the air is not impeded by; the presence of conditions giving rise to wave reflection. Other types'bf sound radiator may be used without" departing. from the spirit of the invention, the principal require ment being that the radiating eificiency should be high at all frequencies inthe desired range.

In the detail description'which follows the invention is described as applied to phone graphs and electrical loud speakers. It will be evident, however, that the principles disclosed may be embodied in ot-her structures, and in acoustic translatingsystems for other purposes, as indicated by the scope of'the appended claims.

R ferring tothe drawings,

Fig. 1 shows in schema-tic form an electrical loud speaker in which; the invention may be embodied;

Fig. 2 is atheoretical diagram representing in conventional form the impedance structure of the system of Fig. l;

Fig. 3 shows typical response characteristics of the loud speaker of Fig. l for diiierentvalues of the horn impedance;

Fig, 4 illustrates diagrammatically a phonograph embodying the invention;

Fig. 5 shows in conventional form the-impedance structure of the system of'E-ig. 4;

Fig. 6 shows diagrammatically typical re sponse-frequency characteristics of'a phono'f graph such as has been regarded as standard in the prior art, and one embodying the invention; v I

Figs. 7 and 8 show in perspective and in cross sectional elevation a complete phonograph machine embodying the invention;

, Figs. 9 and 10 illustrate the detail construction of one form of phonograph sound box in which various features of the invention are embodied; and 5 a v Figsill to 14 illustrate other details of another sound box construction in which additional features of the invention areinvolved.

The diagrammatic representation of an electrical loud speaker in Fig. lillustrates a more or. less ideal arrangement of the vibrae tory system in accordance withthe invention. The mechanical portion-comprises a magnetic armaturel pivoted at itscenter, a diaphragm 3 and an elastic coupling rod 2 connecting the diaphragm to the armature The armature is polarized by. permanent magnet 7, and is caused to vibrate byoscillating currents flowing in coils. 6,,-the driving;force on the armature being proportional to. the intensity of the-current. The acoustical part of the system comprises an air chamber 4, in front of the diaphragm and a tapered horn 5the small end ofwhich is connected to an aperture in the air chamber. The horn may be regarded as an acoustic transmissionlinealong which compressional waves are propagated in the Wellknown manner. At the. mouth-ofthe horn the waves, which-have approximately plane wave fronts, act upon the surrounding air in much the same manner as would a large flat diaphragm or piston vibrating in the mouth of the horn. The air chamber 4 has a two-fold action. In-thefirst place it acts as a velocity transformerby which a relatively small velocity at the surface of the diaphra m is made to producea much larger air velocity in the throat of the horn. This is evident, since all of the air displaced by a movement of the diaphragm naturally tries to escape through the smaller area of. the hornopening. Due to its compressibility, however, the air displaced by the diaphragm does not all escape into the horn, but some part remains in the chamber and increases the air pressure therein. T he air chamber thus acts also as though it were an elastic spring, coupling the diaphragm to the air in the horn. The diaphragm is illustrated as a simple plunger or piston fitting closely into the air chamber. It is free. from any elastic restraint other than what is applied by its coupling to the armature and by the elasticity of the air chamber. For the present purposes the diaphragm is assumed to be sufliciently rigid so that it moves without bending, and so, may be regarded as a simple mass element.

Various Ways by which these conditions mav be closely approximated will be described later.

The mechanical parts together with the air chamber constitute a linked chain of simple mass and elastic elements, or a line of the lumped impedance type. Coupled to this line by the air chamber, in its function as a transforming device, is the homogeneous line formed by the horn, and the terminal load consisting of the free air at the horn mouth.

Before proceeding to discuss the proportioning of the system in accordance with the principles of the invention, the nature of the impedances of the various elements will be consideread-and the impedance values defined. If a-vibratory force is applied to a simple mass element, assumed free from external restraints, the motion of the body is resisted only by its inertia. The body undergoes a vibration synchronous with the force and of such-amplitude that the alternating inertia reactions just balance the applied force. The impedance is directly proportional to the frequency, and since there is no energy dissipation it is purely reactive. The value of the impedance of a body of mass m in grams is equal to jam c. g. s. units where =2n times the frequency, and j is the usual complex operator indicating that the value is imaginary.

When a vibratory force is applied to a spring the force is balanced at all instants by the elastic restoring force due to the displacement of the point of application. The amplitude corresponding to a constant force is. the same at all frequencies, and hence the vibrational. velocity increases in direct proportion to the frequency. Defining the elasticity as the ratio of the total force to the linear displacement at the point of application. and denoting this quantity by s the impedance of an elastic element is equal to .s

In the case of acoustic systems the wave motion takes place under the action of a vibratory pressuresuperimposed on the normal pressure of the air. Acoustic impedance is defined as the ratio of the superimposed pressure intensity to the volumetric rate of displacement of the air past a given cross sectional area of the system. For the present purposes, however, it is more convenient to use the impedance value defined as the ratio of the total pressure over the cross sectional area to the linear velocity of the air particles. This value corresponds directly to the definition of the impedance of the mechanical parts; it will therefore be termed the mechanical impedance to distinguish from the accepted definition of acoustic impedance.

Fig. 2 is an impedance diagram corresponding to Fig 1, in which the nature and the arrangement of the impedance elements are indicated by customary electrical symbols and conventions. The mass elements have m. pa! .gm,

. 4, are shown as shunt, or."coupling, elements. These serve to transmit the force from one line element to the next wit-bout diminution, but'in so doing may abstract some of. the velocity. The line elements,

while transmitting the' vibrational velocity without diminution, tend by their reactions to absorb some ofth'e' impressed force. The transformer '1 represents the transforming property of'the airgclia-mber. The horn is represented. by thetapered line L,, conventionally indicated, and the loadim-pedance R which corresponds to the impedance of the free air as seen from the horn mouth.

The proportioning of the various parts of the-system im accordance with the invention requires that the numerical values of the impedances be known and also the variations of, the impedances with, frequency. Dealing first with the portion comprising the armature and diaphragm sections, this may be regarded aspart of-an extended mechanical line of the lumped impedance type, comprising a chain of mass elements, each of mass m, coupled in tandem by elastic elements, each of'elasticity s. The line is a mechanical broad band wave filter and is adapted to transmit freely waves of all frequencies up to a limiting value 7%, defined by the equation"- 1 f Throughout the greaterpart of this range the characteristic impedance approximates the constant value.

- of special importance. In the first the line terminates in a series element having half the impedance of the regular series element, and in the second the line ends in a shunt clementha 'ing; twice the impedance of a normal shunt element. These terminations, known respectively as -mid-series and ,mid shunt, are unique in that the characteristic impedance is purely resistive at all frequencies in the transmission range. It is generally preferable tomake use of one or other of these terminations when the line is to be connected to a resistive load, or to another line having a resistive characteristic impedance. For this reason the elasticity s, of the air chamber is made equal to twice the normal elasticity of each coupling element of the assumed extended chain, for example, to twice the elasticity 3 of the coupling rod. The mass m, of the armature is coupled to the electromagnetic driving system the impedance of which, as viewed from the mechanical side, is generally negligibly small. Impedance matching at this point is usually impracticable with the ordinary types of electromagnetic system, and, from the standpoint of uniformity of transmission efiiciency, is of lesser importance, provided the impedances of the other portions of the sys-' tcm are matched. It has been found however that advantageous results are obtained by making the armature effective mass approximately equal to 0.8 of the mass of a normal line element. Since the armature motion is an angular vibration about its centre, it is the moment of inertia about this point, rather than the actual mass, that determines the impedance. The mass m, is the effective mass of the armature at the axis of linear motion passing through the centre of the diaphragm.

The foregoing considerations along with Equations 1 and 2 lead to the following explicit formulae for the impedance coeflicients in terms of limiting frequency of the transmission range, and of the characteristic im pedance:

The design of the horn as an cfiicient 'adiator will next be considered. As already stated, the requirement of uniform high radiation etiiciency corresponds to the requirement of a constant resistive impedance at all frequencies in the desired range. In horn radiators two factors are involved in the determination of the radiation efficiency, first the characteristic properties of the horn, regarded as part of an infinitely extended wave transmission line, the impedance coefficients of which are tapered according to a prescribed law, and second, the radiating properties of the mouth. The characteristic impedance of any horn approaches a constant resistance as the frequency rises, but it is found that the constant value isattained most rapidly if the cross sectional area, of the sound conduit varies according to an exponential law with the distance along the axis. tional area, A at a distance a: from' the In a horn of this type the cross-secthroat, is related to the throat area, A by the formula A =A e. (4) The exponential factor 0, is termed the rate of taper and is equal to the fractional increase ef area per unit length along the axis. A unique property of the exponential horn is tha?v its transmissionrange is inherently limited at the low frequency end. An infinitely extended horn of this type would suppress all frequencies below a limiting value 7, given by the equation ac f0 4: 7r in which 0 is the velocity of sound in air. Above this frequency all waves are freely transmitted. I

Above the cut-off frequency defined by Equation 5 the characteristic impedance of the horn has a constant modulus value. At first it is largely reactive but rapidly becomes almost wholly resistive. At twice the cut-off frequency the resistance component has reached 86% of the final steady value and the reactive component is less than 60% of the resistance. The closeness with which the actual impedance of the horn approximates the characteristic impedance depends upon the amount of wave reflection at the mouth, or, in other words, upon the efficiency with which the horn mouth radiates energy to the free air. It has been found that the mouth of a horn is a poor radiator of'vibrations having wave lengths greater than twice its diameter,

the longer waves being largely reflected. The mouth of the horn should therefore be made of such diameter that it can radiate freely waves of the lowest frequency in the desired range. The conduit portion of the horn should be capable of freely transmitting all frequencies that the mouth can radiate. To this end the rate of taper should be such that the wave length corresponding to the cut-off frequency is greater than twice the mouth diameter. The optimum value is found to be numerically equal to 7r, but a range of values from three to four has been found satisfactory and in certain cases the rate of taper may be such that the wave length at the cut-off is as great as six times the mouth diameter without serious loss. When the cutoff wave length is greater than 7r times the mouth diameter the horn is capable of transmitting frequencies lower than the mouth can radiate and the mouth opening is therefore the factor that limits the response range If the horn is straight and circular in cross-section the optimum relationship between the rate of taper and the mouth diameter may be expressed in other terms. For the optimum condition the radius of the mouth opening is equal to 5,

where a is the exponential factor of Equation 4. Also, under this condition, the tangent to the outline of the horn at the mouth makes an angle of 45 degrees with the axis of the horn.

The optimum design is exemplified by a horn of exponential profile having a length of 38.inches, a mouth diameter of 22 inches and a throat diameter of 0.69 inch. For this horn the exponential factor a is 18 25 per cent per inch corresponding to a cut-off frequency of 196 c. p. swor. a wave length of'69 inches.

The frequency at which the mouth'opening begins to fail as a radiator is 300.0. p. s. corresponding to a wave length of 44 inches or twice the mouth diameter. Between 300 c. p. s. and 196 c. p. s. the radiation efficiency falls off graduallyand at lower frequencies practically no sound is reproduced. The frequency equal to 1.5 times the cut-off frequency, which will be termed the pitch limit, may be taken as the limit at which horn assumes its maximum efiiciency of response.

The steady value of the characteristic impedance varies from point to point of the horn in directproportion to the cross sectional area. At the throat it has the value where p is the normal air density, and at the mouth its ,value is increased in proportion to the area. When the hornis proportioned in accordance withthe foregoing rules the condition exists that the horn .impedance is matched to the radiation resistance of the mouthopening, throughout the whole range of efiicient radiationf' The function of theair chamber at the throat of the horn as a transformer isv to match the horn impedance to the characteristie'impedance of the mechanical system. The inclusion of the air chamber transformer in the system'permits the mechanical elements to be designed more or less independently of the horn and to be given practicable proportionssuited to the materials of construction. Where the diaphragm is of the simple plunger or piston type illustrated, the impedance transformation is readily computed. If the diaphragm area be denoted by A then the ratio of the air velocity 'v, at the diaphragm surface to the velocity '0 in the throat of the horn is given by Assuming for the moment that the air in the air chamber is incompressible, then, since there is no absorption of energy in the air chamber, the requirement that the energy imparted to the air by the diaphragm be equal to the energy delivered to the horn, gives rise to the relationship in which Z is the impedanceof the acoustic adiabatic law of expansion.

system as viewed from the diaphragm. The transformation ratio readily follows:

The fact that the air in the air chamber is compressible is taken care of by including the elasticity of the air chamber as an impedance element in the mechanical system.

ratio ofthe increased pressure load on the The air chamberelasticity. is defined as the diaphragm, due to the change of air volume, to the corresponding linear displacement of the center of the diaphragm. To compute its value the air chamber is treated as a comin which V denotes the'air chamber volume, and-in which the other factors have the values already defined.

The foregoing formulae enable the design of the system in accordance with the invention to be completely carried out. The upper limiting frequency of the desired transmission range is animportant factor in deter- .mining the proportions of the mechanical elements, and the lower limiting frequency controls the size and the rate of taper of the horn. Any one of the elements of the me- .chanical system may be chosen as the starting point for the design, but as a rule the practical considerations surrounding the construction of the diaphragm make it most desirable to choose that element as the starting point. The dimensions and mass of the diaphragm having been chosen, the values of air chamber and coupling rod elasticities are readily computed in terms of the diaphragm mass and the upper limiting frequency, by

means of Equation 3. The effective mass m of the armature is directly related to the armature mass. The determination of the actual physical dimensions required to give the desired mass and elasticity coefficients follows well known rules of mechanical design. The volume of the air chamber is determined from the diaphragm area and the desired elasticity by means of Equation 9.

The matching of the horn impedance to the characteristic impedance of the mechanical system remains to be accomplished. The latter impedance is fixed by the foregoing design procedure. The horn impedance is determined by the throat diameter, and may be made any desired value, within limits, by properly choosing the length of the horn. The principle of impedance matching requires that the horn impedance multiplied by the transformation ratio of the air chamber transformer should equal the characteristic impedance of the mechanical system. This is expressed by the equation 2 Z a: Z1!

from which, by Equation 6,

The dimensions of the air chamber in accordance with the requirements of elasticity and impedance transformation may also be related to other impedance coefiicients of the system, and the cut off frequency by additional formulae which are readily derived from the foregoing equations. Thus, the ratio of the air chamber opening areas, in

terms of the cut off frequency and the dia 'phragm mass is given by the equations 1i 7rf,,m J1rf m AfC A. spa. (11) The volume of the air chamber in terms of the horn impedance and the cut off frequency is given by the equations and in terms of the diaphragm mass and the factors entering into the horn impedance by The average depth of the diaphrz m is given by the equation I V pc .21

and is inversely proportional to the mass per unit area of the diaphragm. These equations are useful guides in the design of a matched impedance system, and indicate the manner in which the air chamber dimensions should vary as the design requirements are changed.

In the foregoing formulae 0. g. s. units are assumed; the factor Op entering into the horn impedance has the approximate value 41 at ordinary pressures and temperatures; and the factor 6 in Equation 9 has the value 1.4:X 10".

To illustrate the effect of departure from the matched impedance condition, the re sponse-frequency characteristics of a typical system of the type shown in Fig. 1 have been computed. These are shown in Fig. 3. The ordinates represent the variations of the energy level of the radiation, measured in standard transmission units,-for a constant input force. Logarithmic scales are used for both co-ordinates to correspond with the laws of perception of tone pitch and loudness. Curve "A corresponds to the condition in which the impedance matching is carried out in accordan'ce with the foregoing formulae; curve B shows the effect of making the horn impedance three times ,as great as the impedance of the mechanical. system; and curve C the effeet of reducing the horn impedance to one third of its proper value. The constant input force is adjusted to make the low fre quency response 7 the same in each case. Somewhat similar distortions result if the '1 hornmouth is made too small, or-if the different sections of the mechanical system are not matched to each other.

In establishing the design rules it has been assumed that the diaphragm is of a simple l6 piston type in which all parts move equally with the centre. In actual [practice the diaphragm will be clamped at its edge and will suffer bending of some form as it is vibrated. The bending of the diaphragm has a two-fold effect. In the first place the particles of the diaphragm do not all move with the same velocity and do not contribute equally to the inertia reaction'atithe driving point, and in the second place the elemental surface areas have different displacements and do not con tribute equally to the compression of air in the air chamber. If the law of bending is known, or can be found experimentally, the effective mass of the diaphragm at the driving point can be found by well known analytical methods. The effective area can be determined also by Wellknown methods, this being defined as the area of a rigid piston which. for the same motion of the'driving point, will produce the same air displacement as the flexible diaphragm. Methods for determining the effective mass and effective area of an ordinary' fiat diaphragm clamped at its ed e are described by Hanneman and Hecht, (Sound Fields and Sound Antennae, Engineering. London,February 14, 1919, page 224:) and by Kennelly and Taylor, (Proceedings of the American Philosophical Society, Vol. 54, Mav 1915, pp. 96136) The clamping of the diaphragm also introduces an additional elastic restraint on the motion of the armature which is noticeable particularly at low frequencies. The effect of this restraint is principally to increase the impedance of the system at low frequencies by adding an elastic reactive component that increases as the frequency diminishes. To maintain the response at a high level for the lowest frequencies of speech or music it is desirable that the elastic restraints on the line elements, that. is the series elasticities, should be as small as possible.

The carrying out of the invention in detail will be described in connection with a mechanical phonograph, the vibrating system of which is illustrated diagrammatically in Figs. 4 and 5. The system includes the customary elements, namely, a stylus holder, a diaphragm, and a horn together with an air 65 chamber for coupling between the diaphragm and the horn.- In addition an elastic spider member is provided for coupling between the stylus arm and the diaphragm, thepurpose of this element being to distribute the driving force more uniformly over the surface of the 1 diaphragm. The impedance elements ofthe system are shown in conventional form in Fig. 5, and the corresponding mechanical parts are identified bylike symbols in'Fig. 4. 1

The first part of the system comprises the stylus and stylus arm. The latter is pivoted at an intermediate point and by virtue of its leverage it acts to transform the needle point velocity before impressing it on the diaphragm. In addition to the transforming property, these elements possess mass and elasticity. The elasticity of the needle absorbs some of the velocity imparted to the needle point by the record, and acts therefore as a shunt impedance at the input end. The mass of the needle arm furnishes an inertia reactance, and itselasticity constitutes a coupling impedance in shunt to the remainder of the line. In Fig. 5'the needle arm is represented by the transformer'T mass m 1 and elasticity 8 the mass m being taken as the effective mass of the needle arm at the diaphragm end. This section-corresponds to the armature and coupling rod of Fig. 1, and the impedances are similarly designated. The second part comprises the spider element con sisting of a cupped 'disc with a' number of elastic radial arms the ends of which are attached to the diaphragm. This element is represented by the line mass m correspondioo ing to the solid centre, and the coupling ela'sticity 8 corresponding to the arms. The third section comprises the diaphragm and air chamber combination, represented in Fig. 5

by mass m, and elasticity 8 The fourth part consists of the acoustic load and includes the air chamber in its transformer aspect indicated by T and the radiator impedence Z These are the essential impedance elements of the system. There are, however, additional impedances present due to the elastic restraints introduced bythe methods of supporting and connecting together the essential line elements. The elasticity 8, represents the restraint on the needle arm movement due to the pivot support, elasticity 8 represents the restraint on the spider element due to bending at the point of connection to the needle arm, and 8 represents the elastic restraint on the diaphragm due to the clamping of its edge. By proper mechanical constructions, details of which will be described later the additional series elasticities may be made sufficiently small, so that their effect upon the response characteristic is unimportant. In a 1 system of the type illustrated the most important of the series elasticities is the elasticity of the diaphragm. From the standpoint of wave filter theory, the presence of the series elasticity establishes a lower cut-off frequency at the natural period of the band limit is small, it is desirable that this limit be placed'as low as possible, by reducing to a minimum the series elasticity due to clamping. A convenient check on the design of this portion of the system is providedby determining the two resonance frequencies,

and noting-the interval between them.

The following design calculations illustrate the values of the impedance coefiicients in one practical embodiment of the invention. T

include substantially all of the frequencies of musical tones the limiting frequencies of the transmission range are set at c. p. s. and 5000 c. p. s., the former determining the rate of taper of the horn, and the latter the cut-off frequency of the mechanical system. Talcing the diaphragm as the starting point, the practical considerations relate to obtaining a light structure with sufiicient rigidity to enable it to vibrate as a solid piston. A suitable design consists of a disc of duralumin .0051 ems. (.002) thick and 2.03 cm. (0.8) radius to the'edge of clamping ring. To secure the desired rigidity the diaphragm may be concentrically corrugated over the greater part of its surface, the outer edge being left to provide a highly flexible support. The effec tive inass of this type of diaphragm is nearly equal to its actual mass,'and the effective area to the actual area. Assuming a density of 2.8 for duralumin the diaphragm mass is found to be m =0.186 gins. From Equations-1 and 2 the characteristic impedance of the mechanical system is Z1: rf m,

=2920 c. g. s. units.

-mal value of the coupling elasticity corre sponding to 8 and s is also found from Equations 1 and 2 to be z s 'f =46 10 c. g. s. units.

The air chamber elasticity, in accordance with the rule for terminal impedances should have twice the normal elasticity, or

s,=92 10 c. g. 5. units. The needle elasticity s, as seen from the diaphragm side of transformer T, may have the twice normal value, but it is preferable to use a smaller elasticity than this, approximately equal to the normal value divided by 0.8. This is because the driving source, a record driven by a motor, being powerful enough to impart its velocity to the needle point regardless of the impedance of the system, most nearly corresponds to a source of infinite impedance'and infinite force. There is therefore no consideration of impedance matching and the smaller effective stylus elasticity is found to give a somewhat improred response characteristic at the higher frequencies. The eight-tenths shunt termination is analogous to the eighttenths series termination in the electrical system of Fig. 1. v

The stylus elasticity is generally determined by the type of stylus available. An

ordinary full tone needle may be chosen in ratio to Z -ZE. The proper stylus arm ratio is thus found to be \Vith the chosen diaphragm radius of 2.03

ems. the distance l may conveniently be 2.54 ems. (1 inch) in which case the proper distance from the needle point to the pivot axis would be 2.15 ems (.85 inch).

proper elasticity is found from Equation 9;

the effect-ire area- A, of the diaphragm being 13 square cms. 1f the diameter of the air chamber is the same as that of the diaphragm, the average depth will be 0.182 ems. (0%? inch).

The throat area of the horn required for the proper impedance matching is found by Equation 10 to be 2.4 square cms. corresponding to a diameter of 1.75 cms., or 0.7 inch.

The horn cut-off frequency was set at 100 c. p. s. This corresponds to a rate of taper of 3.05% per cm. of length, or 9.25% per inch ihe volume of the air chamber to give the of length. The diameter of the horn mouth,

assumed circular, should be about one quarter of the wave length at the cut-off frequency; this works out to the 86 ems, or 34 inches. The mouth of the horn need not be circular but if of other shape it should have the same area as the circular mouth of the computed diameter. The length of the horn corresponding to these dimensions'is found to be 84 inches. By increasing the rate of taper very slightly so that the cut-off frequency is 'increasedto 115 cap. s.the horn length may be reduced to 72 inches. Y

7 l-nfthefore oing example, the mechanical system has a relatively 10a characteristic impedance, this being due to theespeciallylight diaphragm construction made possible by the useof a corrugated thin duralumin sheet. For 'a fiat diaphragm of the-ordinary typeit is necessary to use thicker material to obtain a satisfactory degree of rigidity, and in consequence the mass of the diaphragm and the characteristic impedance of the mechanical system are correspondingly increased. To effect the proper matching between the horn and the higher impedance mechanical system resulting from'the use of a heavier diaphragm, it is necessary to reduce the area of the horn throat. in the same proportion aslthe mechanical system impedance is increased. At the same time it is necessary to reduce thevolume of the coupling air chamber in a like proportion, to provide the proper valueof elasticity to cooperate with increased diaphragm mass. For example, assuming a flat diaphragln' of duralumin, (density 2.8) of area 17.8 square ems. and thickness .024 cm.,' (.0095'inch) the design formulae show that for a cut-off frequency of 6,000

= 0.. p. s. the characteristic impedance of the mechanical system is 22,600;c.'g. s. units. The horn'throat area, as found by equation 10, is '.575 cm. ,corresponding to a diameter of .855 cm. or approximately 34 inch. The air chamber volume is found to be .528 cubic centimeters and the average depth to be .03 cms., or .012 inch.

In F 6 curve A represents the response characteristic of a phonograph constructed substantially in accordance with the foregoing dimensions, and curve B that of a recognixed phonograph of the prior art. It will be noted that below 1000 c. p. s., the response of the prior art phonograph falls ofi very rapidly and below 250 c. p. 5. there is practically no response whatever. The characteristic of the phonograph constructed in accordance with the invention shows a uniform response that does not begin to fall off notieeably until the frequency is lower than 200 c. p. s. and is still high atfrequencies as low as 125 c. p. 5. Uniform transmission is maintained down to the limiting frequency defined by the horn mouth diameter, and a fair response is obtained at frequencies down to the horn cut-off. An increased range in the higher frequencies is also obtained and the response is much more uniform throughout the whole range.

A preferred manner of constructing the horn so that, in spite of its considerable length, it may be included in a compact phonograph cabinet, is shown in Figs. 7 and 8. According to this arrangement the complete horn comprises a swinging tone arm 40 coupled to the sound box 42, and a box type folderl horn 41/ The exponential law of the cross sectional areas is carried through both portions. The folded part ofthe horn comprises a mouth or bell portion of rectangular cross section, divided into upper and lower sections 432ml 44 by a horizontal heart shape block 45. Two of the walls of the bell portion may be parallel in which case the desired rate of taper is obtained by adjusting the divergence of the other two walls and the cross sectional shape of the dividing block 45. The back of the bell is curved in conformity with the cross section of the block 45 so that the two sound conduits 43 and 44 are curved forward at the rear to connect with a second pair of horizontal tapered passages cut in the interior of the block. These passages are directed forward through the-block, and, as they narrow down, they are turned backwards again to enter a curved vertical conduit 46, which rises up to the tone arm. The form of the sound passages is shown by the'dotted lines in the perspective view of Fig. 7, and the general mode of construction of the central block is indicated by Fig. 8. The divergent passages carried in block 45 are symmetrical and of equal'length so that in effect they constitute a single passage; In the bell portion the two passages 43 and44 may be of unequal cross sectional areas if desired, but the-axial length of .the passageshouldbe the same in each case. It has been found to be of some advantage with respect to the radiation of the higher frequencies to have one of the passages smaller than the other. There is also the incidental advantage that in case it is desired to place record racks in the mouth of the horn the larger records may be more readily stored in the larger opening. Since the passages converge again at the mouth and are of the same length the whole mouth area is effective for the radiation of low frequencies.

.A practical construction of the mechanical systemis illustrated by the sound box shown in Figs. 9 and 10. This comprises a solid back plate 14, having a central opening and a projecting boss for attachment to the tone arm, and a perforated cover plate 11 to enclose the vibrating elements. The vibrating elements comprisea diaphragm 12 assembled be tween flexible gaskets 13 and clamped between the back plate and the cover by means of the screwed ring or nut-15. The diaphragm is preferably made of light thin material such as thin duralumin sheet and is stiffened by forming in any suitable manner. As shown, it is provided with a number of concentric corrugations of different depths, the outer part being left fiat to keep the elastic restraint due to clamping at a negligibly small value. The diaphragm is driven by a spider element 16, having a hemispherical portion 17, and a plurality. of iexible arms radiating outwardly, and secured to the ridge'of one of the corrugations in any suitable mannerysuch as by riveting, welding, or cementing.

The styius arm 27 at its upper end bears on a projecting boss 28 at the centre of the spider and is .fiexibly tied thereto by a U-shaped spring 26, one arm of which passes through a hole in the boss, while the other engages a nofch in the back of the stylus arm. A socket 30 in the lower end of the arm carries the stylus 32, which is clamped in position by thumb-screw 31. The stylus arm is carried by a bracket 20. attached by screws 19 to the casing 11, and is pivotally supported on downwardly projecting knife edges, 2% and 25, which engage'with grooves in a hardened rod 38 fitted to the arm. The knife edges are formed on a plate 21 attached to the bracket 20. A stiff spring rod 34 also fitted to the stylus arm serves to hold the pivot members in contact. The ends of the spring rod engage in pivot screws 35 and 36 threaded into the turned out ends 37 and 38 of the bracket 20. The holes in these screws are conical, as also are the ends of the rod, and by adjusting the screws some variation of the bearing pres sure can be obtained. The pivot screws are so positioned that the spring support is in line with the knife edges, this being desirable for the purpose of keepin the elastic restraint on the pivot as small as possible.

Figs. 11, 1-2, 13, and 14 show another form of vibratory system and stylusarm mounting especially well suited to the invention. For the sake of clearness, the case .has been omitted. The stylus bar is pivotally supported in ball hearings in a U shaped bracket 51 of magnetic material, attached to the case in any suitable manner. Each arm of the bracket is provided with a tapered ball race opening away from the stylus arm 50 which is supported on a permanently magnetized mounting 52. Each end of the mounting 52 is slightly tapered and the balls 53 forming the bearings are held snugly in place by the n'iagnetizing force of the mounting itself. This force automatically compensates for any wear on the pivots. End plate covers 54 may be used to prevent sudden jars from displacing the balls and to prevent excessive side movement of the mounting.

The upper portion of the stylus arm 50 carried by the mounting 52 is made of thin sheetmaterial having the form of a U shaped channel. The diaphragm is preferably made of a duralumin about .0017 thick and is concentrically corrugated torender its central portion 56 substantially rigid. The clamping edge is fiat and the portion 57 between the clamping edge and the central portion is provided with a series of tapered corrugations which are substantially tangential to the outer corrugation of the central part. The form of the corrugations is indicated by Fig. 13 whichshows the section alon the chord 13, 13 in Fig. 11. It has been Found that this type of corrugation renders the diaphragm very flexible at this point, and reduces any tendency of the edge to resonate within the voice range. The upper end of the stylus bar 50, as shown in Fig..1 1, is flanged and provided with a resilient bifurcated projection 58 extending at right angles to the diaphragm. This bifurcation is notched on the outside near itsend. The diaphragm carries a spider 59 similar to 16 of Fig. 9 with the exception that it has a large central aperture for receiving the bifurcated projection of the stylus arm. The edge of the aperture in the spider engages the notches in the projection of the stylus arm. This provides a self-supporting knife-edge pivot at this point which has negligible series elasticity.

Although the two forms of vibratory systems described herein are especially well adapted to the invention, it is obvious that other forms may be readily devised, the essential points being that the impedance be matched throughout the system and that the serieselasticities be kept as low as consistent with commercial structures.

A sound energy transmissionsystem such ashas been described in which the impedances arematched throughout, will transmit energy in either direction without substantial reflection losses and the phonograph herein described is suited for recording sounds as well as reproducing sounds; It is to be understood that where the term .phonograph or the expression recorder and reproducer is used in the claims, it is intended to define the system of the invention when used either as a sound recorder or reproducer.

The characteristic curves given in Figs. 3 and 6, it will be noted, are plotted in transmission units, (TU), the significance of which is described in an article by W. H. Martin published in the Journal of the American lntitute of Electrical Engineers for June, 1924. The significance of the curves will be better understood when it is realized that the average ear fails to detect variations of less than 5 TU, so that variations in amplitude as great as 8 TU are consistent with high quality of reproduction.

What is claimed is:

1. A sound translating system comprising sound radiating means and a mechanical vibratory system coupled thereto, the masses and elasticities of all of the elements which influence ,the sound wave transmitting properties of said system being proportioned with respect to one another and to the impedance of the sound radiating means, and with relation to the range of frequencies to be transmitted, substantially as described, to give to the system as a whole a response characteristic which is sensibly constant throughout a frequency range of at least 2 octaves withinthe limits of essentialspeech frequencies.

2. A sound translating system according torclaim 1 in which the various elements are proportioned substantially as described to give to the system as a whole a response characteristic which is sensibly constant throughout afrequency range of greater than 3% octaves Within the limits of essential speech frequencies.

3. 'A sound translating system according to claim 1 in which thevarious elements are proportioned substantially as described to give to the system as a whole a response characteristic which varies less than eight transmission' units throughout a range of 2 octaves. 4. A sound translating system according to claim 1 in which the various elements are proportioned substantially as described to give the system as a whole a response characteristic which varies less than fifteen transmission units throughout the range of frequencies between 150 and 1000 cycles.

5. A sound translating system according to claim 1 in which the various elements are proportioned substantially as described to give to the system as a whole a response characteristic which'is sensibly constant throughout a frequency range representing more than one octave in the frequency range below 1000 cycles per second. 6."'An acoustic device in accordance with claim 1 in which the mechanical vibratory system comprises a plurality of substantially uniform sections, each having mass effectively in series and elasticity effectively in shunt. the masses and elasticities being proportioned to constitute the transmission line a broad band wave filter.

7. An acoustic device comprising sound radiating means, and a mechanical vibratory system coupled thereto, the masses and elasticities of the elements of said system being so proportioned with respect to the range of frequencies to be transmitted, and to the impedance of the sound radiating means, that the combination of the vibratory system and the radiating means presents a mechanical impedance to vibrations impressed thereon, which is substantially uniform and resistive throughout a band of frequencies exceeding 2 octaves within the range of essential speech frequencies.

8. An acoustic device in accordance with claim 1 in which the radiating means has a radiating area of such size that its radiation impedance is substantially constant and resistive at all audible frequencies above 200 cycles per second.

9. A sound translating device comprising in combination a horn adapted by its rate of taper and its mouth opening to have a subconstitute a broad band wave filter stantially uniform impedance throughout the essential range of speech and musical frequencies, and a mechanical vibratory system including a diaphragm coupled to said horn, the masses and elasticities of said vibratory system being proportioned and arranged to having a transmission band exceeding 2 octaves in Width within the range of essential speech frequencies, and having a nominal characteristic impedance, as measured at the junction point between the horn and the vibratory system, substantially equal to the horn impedance.

10. In a sound translating device, a horn adapted by its rate of taper and its mouth opening to have a substantially uniform impedance throughout the essential range of speech and musical frequencies, a diaphragm operatively associated with the horn at its small end, and an air chamber disposed between the diaphragm and the horn, the mass of said diaphragm and the elasticity of'the air chamber being proportioned to constitute a portion of a low pass wave filter having a cut-off frequency at least as high as 3000 cycles per second, and the area of the diaphragm being proportioned with respect to the area of the throat of the horn to effect the matching of the horn impedance to the nominal characteristic impedance of the wave filter. j 11. A sound translating device comprising sound radiating means and a mechanical vibratory system coupled thereto,said radiating means having a substantially uniform resistive impedance for sound waves 'of frequencies down to at least 200 cycles per sec- 0nd, and said vibratory system comprising masses and elasticities proportioned and arranged to constitute substantially a low-pass wave filter having a cut-off frequency at least as high as 3000 cycles per second, and having a nominal charactertistic impedance substantially equal to the impedance of the sound radiator.

12. An acoustic horn having an exponential variation of cross sectional area with the length, and having a rate of taper such that the horn transmits freely waves of allfrequencies in the range of efiicient radiation determined by the area of the mouth opening.

13. An acoustic horn having an exponential variation of cross sectional area with the length, and having a mouth opening of diameter approximately equal to one quarter of the sound wave length corresponding to the cut-off frequency determined bythe rate of taper of the horn.

' 14. An acoustic horn having an exponential variation of cross sectional area with the length, and having a mouth opening of area approximately equal to the area of a circle the diameter of which is equal to one quarter of the sound wave length corresponding to the cut-oil frequency. determined by the rate of taper of the horn.

15. A horn for sound transmission and radiation having a rate of taper at least as small as 20% increase of area per inch of length over a substantial portion of its length, and a mouth opening havingan area .equiva? .lent to that of a circle of diameter substantially equal to one-fourth of the wave length corresponding to the cut-off frequency determined by. said rate of taper. 1

16. A horn for sound transmission and radiation corresponding to claim 15 in which the rate of taper is at least as small as 15% increase of area per inch of length over a substantial portion of its length.

17. A horn for sound transmission and radiation corresponding to claim 15 in which the rate of taper is at least as small as 10% increase of area-per inch of length over a sub stantial portionof its length.

18. A sound translating system comprising an acoustic horn having a substantially constant impedance throughout the range of essential speech frequencies, a diaphragm and driving means therefor located at the small end of said horn, and an air chamber between said diaphragm and horn, said diaphragm, driving means, and air chamber, being proportioned with respect to their masses and elasticities to constitute a transmission line of substantially uniform characteristic impedance throughout therange of essential speech frequencies, and the area of the diaphragm being proportioned with respect to the area of the throat of the hornto effect the matching of the horn impedanceto the characteristic impedance of said line.

19. In combination, an acoustic horn having a substantially constant :impedance throughout the essential range of speech frequencies, a vibratory system including a diaphragm located at the small end of said horn, and an air chamber coupling said diaphragm to the horn, the area of the diaphragm being proportioned with respect to the area of the opening from said air chamber to the horn, to efiect the matching of the horn impedance to the characteristic impedance of the vibratory system.

20. In combination, an acoustic horn, a vibratory system including a diaphragm located at the small end of said horn, and an air chamber coupling said diaphragm to the horn, said air chamber being proportioned with respect to its elasticity to co-operate with the masses and elasticities of said vibratory system in providing a mechanical wave transmission line of substantially uniform characteristic impedance throughout the essential range of speech frequencies.

21. In combination, an acoustic horn, a vibratory system including a diaphragm located at the small end of said horn, and an 5 air chamber coupling said diaphragm to the awe e horn, the masses andhelastic ities of the elements of said vibratory system and said air chamber being proportioned and arranged to provide a multi-resonant system, having natural periods of vibration distributed in a regular manner-throughout the range of essential speech frequencies, and said horn having a throat area at its point of connection to said airjchambe r such that the horn im pedance provides substantially .nniforni damping of the vibrations of said .sy stemat all frequencies in the resonance-range.

22. An acoustic device having a :hornand a diaphragm coupled thereto by an aipchamher, the ratio of the equivalent plunger area throat opening being mun e rical lygreater than the square root of tl1e. 1itl0:( )f. l0OOO times the effective mass of thediaphragm in grams, to l1 timesthe area of the horn throat opening. infsquare centimeters. 5 I

23. An acousticginstrument having adia- 'p m a dra horn, th rdi l m m hav n ma in grams, P q are en im t ri i area-at st sm l w ndyhre teahousandths ofthe ,ratio of the aneapftthesmall end of the horn to the equivalent plunger area of he d p a 24. An acoustic idevice comprisinga horn, a vibratory system a. diaphragm located at the small end of said hor11;,,and;an air chamber coupling said, diaphragm and said horn, the-ratiolof the etfective area of the diaphragm to' the area- ,o fsthe throat of the horn being numerically greater than 250 times the quotient ofi themfiective mass of the diaphragm in grams, dividedby the efiec tive area vof; the diaphragm square centimeters gm- 25. An acoustic device comprising a born,

a vibratory system including a diaphragm located at the small end of said horn, and an air chamber couplingsaid diaphragm and said horn, the volume of said ,air chamber in cubic: centimeters being numerically equal to the product of the effective mass of. the diaphragm in grams, and the square of the ratio of the horn throat area to the et'fectiye area of the diaphragm, multiplied by a numerical factor lying between the values 340 and 420.

26. An acoustic device comprising? a -horn, a vibratory system including a diaphragm located at the smallend=of said horn, and an air chamber couplingisaid diaphragm and said horn, said air chamber havingan area substantially equal to that of said diaphragm, and an average depth, in centimeters, which is numerically less than seven tenthousandths divided by'the mass in grams per square centimeter of area of the diaphragm. v

27. A system in accordance with claim 1 in which the ratio of the equivalent plunger area of the diaphragm to the area of the horn throat opening is numerically greater than the square root of the ratio of 10,000 times '80 of said diaphragm to the area of the 'horn the effective mass of the diaphragm in grams, to 41 times the area of the horn throat opening in square centimeters.

28. A system in accordance with claim 19 in which the ratio of the effective area of the diaphragm, to the area of the horn throat opening, is numerically greater than 250 times the quotient of the effective mass of the diaphragm in grams, divided by the effective area of the diaphragm in square centimeters.

29. A system in accordance with claim 19 in which the volume of the air chamber in cubic centimeters is numerically equal to the product of the effective mass of the diaphragm in grams, and the square of the ratio of the horn throat area to the effective diaphragm area, multiplied by a numerical factor lying between the values 340 and 420.

30. A system in accordance with claim 19 in which the air chamber has an area substantially equal to that of said diaphragm, and has an average depth, in centimeters, numerically less than seven ten-thousandths divided by the mass in grams per square centimeter of area of the diaphragm.

31. A system in accordance with claim 19 in which the volume of the air chamber in cubic centimeters, is numerically less than 1.7 times the area of the horn throat opening in square centimeters.

32. In an acoustic instrument, the combination of a horn, a diaphragm, and an air chamber coupling said diaphragm and said horn, in which the connecting opening between said horn and said chamber is approximately .7- inch diameter and the diameter of the equivalent plunger area of the diaphragm is approximately 1.6 inches, and the air chamber volume between the diaphragm and horn opening is at least as small as one-fifth cubic inch. i

33. Anacoustic instrument having a sound radiator such as a horn, and a vibratory system including a diaphragm co-operating with said sound radiator, the elements of said vibratorysystem constituting a mechanical transmission line having series masses and having both shunt and series elasticities, the sum of all of the series elasticities being at least as small as six million dyncs per centimeter.

34. An acoustic device comprising a horn, a diaphragm located at the small end of the horn, and an air chamber coupling said diaphragm and said horn, said diaphragm being supported at its edge and being resonant at a frequency lower than 600 c. p. s. due to its own mass and elasticity, and the elasticity of the said air chamber being such as to make the comb'ination of the air chamber and diaphragm resonant at a frequency higher than 2000 cycles per second.

35. An acoustic device comprising a horn, a diaphragm located at the small end of said horn, and an air chamber coupling said diaphragm to the horn, said diaphragm being supported at its edge by the air chamber enclosure, and the resonance frequency of the diaphragm, due to its own mass and the elastic restraint of its support, being separated from the resonance frequency of the diaphragm in combination with the air chamber by an interval of at least 1400 cycles per second.

36. An acoustic device comprising a horn, a diaphragm located at the small end of said horn, and an air chamber coupling said diaphragm and said horn, the effective mass of said diaphragm being restrained in its motion by a series elasticity due to its support, and being coupled to the horn by the shunt elasticity of the air chamber, said series elasticity being such as to resonate-with the effective mass of the diaphragm at a frequency lower than 600 cycles persecond, and said shunt elasticity being suchas to increase the resonance frequency of the diaphragm to a value greater than 2000 cycles per second.

37. An acoustic phonograph having a horn, the effective length of which'is more than 40 inches, and having matched impedance from the stylus point to the mouth of saidhorn throughout the essential speech frequency range. I

38. A phonograph comprising a sound box, and a horn having substantially matched impedance from the styluspoint to free air throughout the essential speech frequency ng J v39. Incombination aphonograph reproducer and recorder having matched impedance from the point of the stylus to the diaphragm inclusive, throughout the essential speech freqency range, and a horn having an effective length of at least 40 inches, and a mouth opening to free air the diameter of which is approximately one-fourth the wave length of the lowest frequency to be trans mitted.

40. A sound recorder and reproducer having a vibratory member and astylus arm coupled thereto, a pivot support for said stylus arm, and a needle attached to the lower portion of said arm, in which the ratio of the distance from said needle point to the axis of said support, tothe distance from the axis to the point of attachment to the vibratory mem ber is approximately equal to the square root of the ratio of the needle point elasticity to eight-tenths the elasticity of the needle arm ti 11. A sound recorder and reproducer having a stylus arm in which the square root of the ratio of the stylus arm tip elasticity to the stylus arm mass is greater than 10,000, a diaphragm, and a coupling means between said stylus arm tip and said diaphragm, having a ratio of shunt elasticity to series mass of the same order as the first mentioned ratio.

42. In a sound reproducer and recorder having a stylus arm, an air chai'nher and a dia phragm therein, means connecting said stylus arm and said diaphragm, said means having substantially the same characteristic impedance as the combination of said diaphragm and a chamber throughout the range of frequencies of importance in speech and music.

43. A sound recorder and reproducer having a diaphragm and a casing therefor, said casing and diaphragm forming a chamber having a zigzag cross section.

4.4. A sound recorder and reproducer according to claim 43, wherein the radial cross section of the chamber increases progressively in thickness towards the center.

45. In a sound recorder and reproducer, a corrugated diaphragm, all flat spaces of which have their natural frequencies above the voice frequency range.

46. A sound recorder and reproducer comprising a stylus bar, a concentrically corrugated diaphragm, a spider connecting said bar and said diaphragm, said spider having a stiff central portion and flexible radial arms, the stylus bar being connected to said central portion, and the ends of said arms being rigidly secured to a ridge of one of the corrugations of said diaphragm.

47. A sound recorder and reproducer acear-dine, to claim 46, wherein the corrugations of said diaphragmare of unequal altitudes.

48. A sound recorder and reproducer according to claim 46, wherein the ratio of elas ticity to mass of each corrugation is of such value as to give a critical frequency value above the range of importance in music and speech.

49. A sound recorder and reproducer having a corrugated diaphragm and a casing having a corrugated surface facing which matches the corrugations of said diaphragm.

50. A sound recorder and reproducer according to claim 49, wherein the corrugations in both diaphragm and casing are concentric.

51. A sound recorder and reproducer having a corrugated diaphragm and a casing forming a chamber, said casing having a corrugated surface facing, and being so disposed with respect to said diaphragm that the velocity of the vibrations are substantially constant throughout the chamber.

52. A sound'recorder and reproducer having a stylus arm and a diaphragm, means connecting said stylus arm and said diaphragm comprising a substantially rigid cup-shaped portion connected to said arm, and a plurality of elastic arms extending substantially radially from said cup portion connected to said diaphragm.

53. A sound recorder and reproducer according to claim 52, wherein the stylus arm malres pivotal contact with the cup portion of said connecting means.

54. A sound recorder and reproducer havstylus arm, said stylus arm having a flanged 0nd yieldingly engaging said vibratory memher in the perforation thereof.

50. A sound recorder and reproducer comprising a vibratory member, and a channelshaped it iy member.

57. A sound recorder and reproducer com prising a stylus bar, ball bearing pivots therefor, and magnetic means for maintaining the adjustment of the balls in said pivots.

58. In combination, a diaphragm, a dia- ;:-hragm chamber enclosing the same, a horn, the cross-sectional area of which increases by equal percentages for equal increases of distance from the small end, the diaphragm chamber opening into the small end of the horn, the rate of increase of said cross-section, and the cross sectional area of thesinall end being such that the ratio of the damping of the diaphragm by said born to the mass of the diaphragm is greater than 10,000 c. g. s. units.

59. In an acoustic instrument, a diaphragm chamber, a diaphragm therein, a horn, the cross section of which is an exponential func tion of the distance from the small end, the small end of said horn communicating with said diaphragm chamber, whereby the acoustic damping upon said diaphragm will be a function of the area of said small end of the horn, said area being so small that the acoustic damping per unitmass of the diaphragm is siniicient to prevent the ratio of sound energy radiated at the natural resonance period of the diaphragm to the sound energy radiated at the lowest frequency at which the horn can effectively radiate to be less than a normal ear can distinguish when listening to music.

(30. in an acoustic instrument, a diaphragm chamber, a diaphragm therein, a born, the cross section of which is an exponential funct on of the distance from the small end, the small end of said horn communicating with said diaphragm chamber, whereby the acoustic damping upon said diaphragm will be a function of the area of said small end of the horn, said area being so small that the acoustic I damping is suflicient to prevent perceptible distortion over the range of frequency for which the horn is intended. I

61. In an acoustic instrument, a diaphragm chamber, a diaphragm therein, a horn, the

said stylus arm and said diastylus arm having a bifurcated flanged end yieldingly engaging said vibra cross section of which is an exponential function of the distance from the small end, the small end of said horn communicating with saic diaphragm chamber, whereby the acoustie damping upon said diaphragm will be a function of the area of said small end of the horn, said area being so small that the acous tic damping is sufiicient to cause the radiated sound to be substantially independent of the frequency from the highest frequencies for which the instrument is intended to the cutoil frequency determined by the rate of increase of said horn.

62. In combination, a diaphragm, a horn, an enclosure housing said diaphragm and opening into said horn, said opening being as small, relative to 'the volume of said enclosure, as may be without causing the volumetric flow of air into said opening to be appreciably less than the volumetric rate of displacement of the diaphragm.

63. In combination, a diaphragm, means for vibrating said diaphragm, and acoustic means for so dampin said vibration that the displacement. of the diaphragm does not ex cecd an amount at which the restorative forces thereof cease to have a linear relation to said displacement.

64. In combination, a sound-reproducing device, a horn for acoustically damping the same, said horn having a crosssection which is an exponential function of the distance from the small end and having a hell with walls making a maximum angle of forty-five degrees with the axis, and a throat, the minimm diameter of which is so small that the acoustic damping is greater than the sum of all other damping.

65. In a sound-reproducing instrument, a horn, a diaphragm, an enclosure providing a chamber between said diaphragm and horn, said chamber opening into the throat of said horn. the area of said opening being large enough relative to the area of said diaphragm and volume of said chamber, to ensure the VOlllll'lG-llliC flow of air into the opening shall be equal to the volumetric rate of displacemrnt of the diaphragm. and small enough to damp the motion of said diaphragm more than the sum of all other damping.

(36. An acoustic horn. having an exponential variation of cross-section with the length and an internal diameter at the small end less than two per cent of the internal diameter at the large end.

67. An acoustic horn, having an exponential variation of cross-section with the length, the rate of increase of cross-section per unit length being suilicicntlv small to bring the pitch limit of accurate reproduction within an octave of middle C.

68. An acoustic horn, the area of whose cross-section is an exponential function of the distance from the small end, the rate of increase of the cross-section being less than twenty-five per cent per inch and the diameter of the opening at the small end being less than two per cent of the diameter of the opening at the large end.

j {59. In a sound-reproducing instrument, a diaphragm, a horn, a housing over said diaphragm opening into said horn, the height of said housing being less than fifty times the maximum amplitude of the motion of the diaphragm when loaded by said horn.

70. In a sound-reproducing instrument, a chamber having an opening, and means cooperating with said chamber to produce an air movement in said opening corresponding to the sound to be reproduced, the area of said opening being such that the pressure 'ithin the chamber is maintained substantially constant.

71. In combination, a diaphragm, a horn, an enclosure housing said diaphragm and opening into said horn, said opening being so proportioned, relative to the volume of said enclosure, that the volumetric flow of air through said opening will be substantially equal to the volumetric rate of displacement of the diaphragm.

72. In an acoustic instrument a diaphragm chamber, a diaphragm therein, a horn, the cross-section of which is an exponential function of the distance from the small end, the small end of said horn communicating with said diaphragm chamber, whereby the acoustic damping upon said diaphragm will be a function of the area of said small end of the horn, said area being so small that the acoustic damping per unit mass is greater than (a, 3601 quency of the diaphragm and (n is the lowest frequency which the instrument is expected to radiate.

73. In an acoustic instrument, a horn having a cross-section which is an exponential function of the distance from the small end, the rate of increase of said cross-section being less than twenty-five per cent per inch and the area of the large end being at least equal to that of a circle having where a is the constant in the exponent. determining said rate of increase.

74:. In an acoustic instrument, a horn having a cross-section which is an exponential function of the distance from the small end, the area of the large end being greater than the area of a circle having a diameter equal to one-sixth of the wave length of sound at the frequency of the lowest natural resonance pitch of the horn.

75. An acoustic horn having an exponential variation of cross-sectional area With the length and having a mouth opening of area equal to the area of a circle the diameter of Where w, is the natural resonance frefor its radius,

which lies between onefifth and one third of the sound Wave length corresponding to the cut-elf frequency determined by the rate of taper of the horn.

in witness whereof, I hereunto subscribe my name this 8th day of October, A. D. 1927.

HENRY C. HARRISON. 

